Decades ago so-called paired- or coupled-pacing was used, primarily in the context of experiments and studies. This type of pacing involves the delivery of an extra-systolic stimulus (ESS), in effect a pacing-level pulse of electricity, to a chamber of the heart a relatively short interval after a paced or intrinsic depolarization of that chamber. The ESS pulse is applied following the refractory period after an initial paced or spontaneous depolarization, and results in a second electrical depolarization of the chamber substantially free of an attendant myocardial contraction. The second depolarization of the chamber effectively prolongs the refractory period after the mechanical contraction of the chamber caused by the first paced or intrinsic depolarization.
The prolonged refractory period caused by ESS therapy delivery effectively slows the heart rate from its spontaneous rhythm. Consequently, the prolonged refractory period allows a greater time for filling of the chamber. Further, ESS therapy delivery causes an augmentation of contractile force of the chamber following the cardiac cycle during which an ESS pulse was applied. If no additional ESS pulses are applied in subsequent cardiac cycles the magnitude of the augmentation attenuates over the next few cycles. The combination of increased filling and contractile force augmentation provides an immediate boost to stroke volume and, under certain circumstances, can lead to increased cardiac output. For this reason, ESS therapy delivery has been proposed as a therapy for patients with congestive heart failure (CHF), left ventricular dysfunction (LVD), cardiac insufficiency, post-resuscitation pulse-less electrical activity (PEA) or electro-mechanical dissociation (EMD), and the like.
On the other hand, delivery of ESS pacing pulses—particularly of relatively high magnitude—if delivered too close to or during the refractory period (e.g., the vulnerable period), can provoke an arrhythmia episode. As a result, delivery of ESS therapy, particularly to patients with CHF, LVD, PEA, EMD and/or cardiac insufficiency who may be more susceptible to an episode arrhythmia than the general population, must be carefully controlled with caution and using state-of-the-art pacing platforms.